Inhibition of p53 function, either through mutation or interaction with viral transforming proteins, correlates strongly with the oncogenic potential of the cell. Only a small percentage of human T-lymphotropic virus type-I (HTLV-I)-transformed cells carry p53 mutations and mutated p53 genes have been found in only one-fourth of ATL cases. In previous studies, we demonstrated that wild-type p53 is stabilized and transcriptionally inactive in HTLV-transformed cells. Further, the viral transcriptional activator Tax plays a role in both the stabilization and inactivation of p53 through a mechanism involving the first 52 amino acids of p53. Here we show for the first time that phosphorylation of p53 inactivates p53 by blocking its interaction with basal transcription factors. Using two-dimensional peptide mapping, we demonstrate that peptides corresponding to amino acids 1-19 and 387-393 are hyper-phosphorylated in HTLV-I-transformed cells. Moreover, using antibodies specific for phosphorylated Ser15 and Ser392, we demonstrate increased phosphorylation of these amino acids. Since HTLV-I p53 binds DNA in a sequence-specific manner, but fails to interact with TFIID, we tested whether phosphorylation of the N-terminus of p53 affected p53- TFIID interaction. Using biotinylated peptides we show that phosphorylation of Ser15 alone inhibits p53-TFIID interaction. In contrast, phosphorylation at Ser15 and 37 restores TFIID binding and blocks MDM2 binding. Our studies provide evidence that HTLV-I utilizes the post-translational modification of p53 in vivo to inactivate function of the tumor suppressor protein.We have also studied the effect of p53 post-translational modification on interaction with cellular regulatory proteins. p53 exerts its cell cycle regulatory effects through its ability to function as a sequence-specific DNA binding transcription factor. CBP/p300, through its interaction with the N-terminus of p53, acts as a coactivator for p53 and increases p53s sequence-specific DNA-binding activity by acetylating its C-terminus. The same N-terminal domain of p53 has recently been shown to be phosphorylated at Ser15 in response to g-irradiation. Remarkably, we now demonstrate that phosphorylation of p53 at Ser15 increases its ability to recruit CBP/p300. The increase in CBP/p300 binding was followed by an increase in the overall level of acetylation of the C- terminus of p53. These results provide a mechanism for the activation of p53-regulated genes following DNA damage, through a signalling pathway linking p53 N-terminal kinase and C-terminal acetyltransferase activities. - Cytokines, DNA binding proteins, Gene regulation, Leukemia, p, Tumor Suppressor, Phosphorylation, Retroviruses, Virus- Cell Interactions,